Comtech EF Data MD2401 - Manual (Page 48)

MD2401 L-Band Multi Demod Installation and Operation Manual User Interfaces
MN-MD2401 4–16
Revision 7
4.5.7 Flow Control and Task Processing
The original packet sender (the M&C computer) relies on accurate timeout information with
regard to each piece of equipment under its control. This provides for efficient bus communication
without unnecessary handshake overhead timing. One critical value is designated the Inter-
Frame Space (FS). The Inter-Frame Space provides a period of time in which the packet receiver
and medium (control bus and M&C computer interface) fully recover from the packet
transmission/reception process and the receiver is ready to accept a new message. The
programmed value of the Inter-Frame Space should be greater than the sum of the “turnaround
time” and the round-trip (sender/receiver/bus) propagation time, including handshake overhead.
The term “turnaround time” refers to the amount of time required for a receiver to be re-enabled
and ready to receive a packet after having just received a packet. In flow control programming,
the Inter-Frame Space may be determined empirically in accord with the system configuration, or
calculated based on established maximum equipment task processing times.
Each piece of supported equipment on the control bus executes a Radyne Link Level Task
(RLLT) in accordance with its internal hardware and fixed program structure. In a flow control
example, the RLLT issues an internal “message in” system call to invoke an I/0 wait condition that
persists until the task receives a command from the M&C computer. The RLLT has the option of
setting a timeout on the incoming message. Thus, if the equipment does not receive an
information/command packet within a given time period, the associated RLLT exits the I/0 wait
state and takes appropriate action.
Radyne equipment is logically linked to the control bus via an Internal I/O Processing Task (IOPT)
to handle frame sequencing, error checking, and handshaking. The IOPT is essentially a link
between the equipment RLLT and the control bus. Each time the M&C computer sends a
message packet, the IOPT receives the message and performs error checking. If errors are
absent, the IOPT passes the message to the equipment’s RLLT. If the IOPT detects errors, it
appends error messages to the packet. Whenever an error occurs, the IOPT notes it and discards
the message; but it keeps track of the incoming packet. Once the packet is complete, the IOPT
conveys the appropriate message to the RLLT and invokes an I/0 wait state (wait for next
character).
If the RLLT receives the packetized message from the sender before it times out, it checks for
any error messages appended by the IOPT. In the absence of errors, the RLLT processes the
received command sent via the transmitted packet and issues a “message out” system call to
ultimately acknowledge the received packet. This call generates the response packet conveyed to
the sender. If the IOPT sensed errors in the received packet and an RLLT timeout has not
occurred, the RLLT causes the equipment to issue the appropriate error message(s) in the
pending equipment response frame.
To maintain frame synchronization, the IOPT keeps track of error-laden packets and packets
intended for other equipment for the duration of each received packet. Once the packet is
complete, the IOPT invokes an I/0 wait state and searches for the next character.
4.5.8 RLLP Summary
The RLLP is a simple send-and-wait protocol that automatically re-transmits a packet when an
error is detected, or when an acknowledgment (response) packet is absent.
During transmission, the protocol wrapper surrounds the actual data to form information packets.
Each transmitted packet is subject to time out and frame sequence control parameters, after
which the packet sender waits for the receiver to convey its response. Once a receiver verifies